The present invention relates to control of compensating impedance connected to supply leading or lagging reactive current to an electric power system for stabilization of voltage, and particularly to a control system in which regulation stability is enhanced under no load or light load conditions.
It is known that electric power systems which supply highly erratic reactive loads, e.g., electric arc furnaces, are typically characterized by poor voltage regulation. Consequently, these systems often exhibit an undesirable flicker. One system suggested to alleviate this condition is disclosed in U.S. Pat. No. 3,936,727, issued to Kelley et al, on Feb. 3, 1976, which is hereby incorporated by reference in the present application. The Kelley et al patent is assigned to the assignee of the present application.
The Kelley et al system is generally illustrated in FIGS. 1 & 2. Briefly, the system includes the use of two control loops: one open compensating control loop; and one closed regulating or supervisory control loop. The compensating control loop senses the reactive load current component and attempts to negate the same by providing the appropriate compensating reactive current component through networks designated CR and IR. The regulating or supervisory control loop employs a current angle sensor 70 to sense the power factor or phase angle at a selected line location which is typically located at a critical area of the line at which good voltage regulation is desired. In the case of a typical arc furnace load, good voltage regulation would involve less than about 1/4% modulation of the magnitude of the 60 cycle voltage waveform. After sensing the magnitude of the phase angle at the critical line area, the supervisory control loop produces an output signal which is representative in magnitude of the phase angle as a function of time, as shown in FIG. 2c. This output signal (FIG. 2c) is dynamically compensated for stability of the supervisory regulating loop under normal gain conditions by the stabilizing circuit 85 of FIG. 1 and then acts through a path common with the compensating loop involving the IR and CR networks of FIG. 1 so as to ensure that the system remains properly compensated over a long time period.
Generally, the system shown in FIGS. 1 and 2, and more completely described in the Kelley et al patent, satisfactorily minimizes the problem presented by the erratic reactive load. However, under conditions of no load or light load, i.e., arc furnace turned off, the regulating system, or loop tends to become unstable. This tendency toward instability under no load or light load conditions is due to the fact that, under these conditions, a slight excess of, or deficiency of, inductive current is measured by the current angle sensor 70 as a phase angle of either -90.degree. or +90.degree.. Under these conditions, where zero phase angle regulation is desired at the critical line area, the supervisory control loop produces output signals representative of .+-. 90.degree. phase angle conditions and directs the IR and CR networks of FIG. 1 to produce the appropriate compensating current. This results in a situation in which the phase angle to be regulated will swing through zero very rapidly with only a small adjustment of the compensating current. For example, when the supervisory control loop output signal indicates that the phase angle is + 90.degree., a small increase in inductive compensating current causes the phase angle to rapidly swing through zero, resulting in a phase angle of -90.degree.. This oscillating process continues so as to make zero phase angle regulation difficult to achieve. Further, this rapid variation may itself cause disturbances which result in poor voltage regulation. The undesirable situation in which a relatively small adjustment of compensating current causes a relatively large variation of phase angle at the critical line area can be conveniently referred to as one in which the regulating control loop exhibits excessively high gain.
An object of this invention is to develop a power system in which satisfactory voltage regulation is provided under no load, light load, and highly erratic reactance load conditions.
Another object of this invention is to develop such a power system which allows for stable regulation at other than zero phase angle.